1. Field of the Invention
This invention relates to a process for the manufacture of microfiltration membranes with pore sizes of from 0.05 to 10 um. by dissolving a copolymer of isophthalic acid and m-phenylenediamine in a solvent mixture of dimethylacetamide and dimethylformamide in the presence of alkali or alkaline earth chlorides and ethyleneglycol, and pouring out the solution onto a smooth surface formed of chemically inert material with the formation of the membrane, a relative air humidity being maintained.
2. Description of the Related Art Including Information Disclosed Under 37 CFR .sctn..sctn.1.97-1.99
A process of the initially-mentioned type is disclosed in DE-PS 32 20 376. In the case of this known process the desired pore size of the membranes is adjusted by the proportion of solvent used, at least two of the solvents known as dimethylacetamide, dimethylformamide or N-methylpyrrolidone being used. The maximum content for each individual solvent is 80% of the total content of solvents.
Furthermore, in accordance with this known process, it is extremely important to add water with 1 to 4 parts per weight of the solution and 10 to 25 parts by weight of a polyvalent alcohol and/or water as a structure-loosening additive. Methyleneglycol or ethyleneglycol are used as polyvalent alcohols. In this connection it has proved to be advantageous to add already to the aforementioned alcohols the amount of water, or, even better, the corresponding solvents, dimethylacetamide, dimethylformamide or N-methylpyrrolidone which are used to dilute the polyamide solution in order to avoid irreversible precipitation.
When these known processes are used it has been shown that membranes with a pore size of from 0.05 um. to 10 um. can be produced but that in particular membranes with a pore size between 0.05 and 0.65 um. cannot always be produced in a reproducible manner.